At most large American airports, collecting a rental car means a train or bus to a separate building. The arrangement is deliberate, and it replaced something worse.
Curbside space is the scarcest asset
The roadway outside a terminal has a fixed length, and every vehicle type competes for it: private cars, taxis, ride-hailing, hotel shuttles and buses.
When each rental company ran its own shuttle, a large share of that curb and of the access roads was occupied by mostly empty buses circulating continuously.
Removing them frees curb frontage for uses that cannot be relocated, which is the central reason consolidation happens.
One facility replaces many operations
A consolidated center puts all the companies under one roof with shared roadways, fueling, washing and storage, so the vehicle servicing is done once rather than at a dozen scattered lots.
Land near a terminal is extremely valuable, and rental fleets need large flat areas, so moving them outward releases ground for aviation uses that must be adjacent to the airfield.
The building is usually connected by a dedicated shuttle or an automated train, which is far more efficient per passenger than many individual buses.
The facility is financed by its users
These centers are expensive, and they are typically funded through charges collected from rental customers rather than from general airport revenue.
That is why rental agreements at airports carry separate line items beyond the base rate and the usual taxes.
The charge continues after construction because the facility has operating costs and long-term debt attached to it.
The trade is time against reliability
Travelers exchange a walk for a ride, which adds minutes to the arrival. In return the transfer is frequent, weather-protected and predictable in a way individual shuttles were not.
Return journeys benefit most, since a driver heading to a flight follows clear signage to one destination rather than searching for a company-specific lot.
The design changes how you should plan
Building the transfer into the schedule matters more for departures than arrivals, because the return, refueling and shuttle all sit before check-in.
At airports where the center serves several terminals, the train may stop at each in turn, and the last stop can be a substantial addition to the trip.
Allowing for that transfer is the practical difference between an unhurried return and a sprint through security.